What is anti-skating?

Lately we’ve noticed the release of several turntables—some of them otherwise high-end, expensive units—that dispense with the traditional anti-skating feature.

Sadly, we’ve also read various reasons why it wasn’t necessary on those turntables, and these reasons typically betray a misunderstanding of why it’s needed in the first place.

n fact, anti-skating is needed on virtually all turntables.

he requirement for anti-skating is nothing to do with the quality of the tonearm’s construction or the make of the tone arm bearings or whether the arm is straight or curved. It’s all to do with the directions of forces.

The only exceptions to the rule are those turntables which have radically different tone arm geometry.

It’s All to do with the Directions of the Forces

When the stylus is in the groove of a record, there is friction between it and the walls of the grooves. That means that the vinyl recording is tugging on the stylus. How hard the tugging is depends on several factors.

One is the precise formulation of the vinyl and the stylus. Different materials can have a slightly different ‘coefficient of friction’ between them. Another is the modulation of the record. Wildly swinging grooves are going to tug a little harder than almost straight grooves. These are things over which the listener has no control.
One other factor affecting friction is the tracking weight. This is a very important factor. We all learned Amonton’s First Law in High School Science, didn’t we? It says: ‘The force of friction is directly proportional to the applied load.’ Double the tracking force and you double the friction.

But you’ll probably be choosing your cartridge/stylus combination only partly on the criterion of tracking force. We all know that you should stick with the manufacturer’s recommended tracking force, absent an extremely good reason to depart from it. In particular, reducing the force is likely to result in more damage to the record due to mistracking.

So, we’ve established we have a force on the stylus. In what direction is that force? It’s aligned with the tangent to the groove in which the stylus is implanted at any particular moment. The direction of that tangent can vary over a few degrees as the arm tracks across the record from the edge towards the centre, but not by much.

At this point a picture becomes useful. That force is in the direction ‘B’ shown in the graphic below.

The following is going to seem like a foolishly trivial question and answer session, but they’re important to understanding how the forces interact.

First: What is it that stops the stylus from simply going around and around with the record?

Second: What keeps it in place on the end of the tone arm? Mr Newton says it would go with the record groove unless there’s an equal and opposite force... and of course there is.

Where’s The Pivot?
Most of that opposite force is supplied by the tone arm’s pivot. Well, technically, the force opposing that friction is supplied by the stylus cantilever. The force holding the cantilever in place is provided by the cartridge… then the tone arm, and then the tone-arm’s pivot. You can trace it all the way back to the planet itself, but the pivot is the sensible stopping point because the pivot is, in fact, a pivot.

That is, because it can swing freely from side to side on low-friction bearings, the only force that it can apply to anything on the horizontal plane is either directly towards the pivot or directly away from it. And here’s the problem. The pivot is not located on a tangent to the groove. It is to the left of that tangent for almost all extant tonearm designs. That means that the force it applies is not directly opposite the force applied to the stylus by the groove. You can see that in the graphic, where I have labelled it ‘C’. The pivot cannot apply any significant sideways force because it is low in friction.

But Mr Newton insisted that the force had to be opposite. So there is something called a ‘residual’ force on the stylus, which is the force left after the force from the pivot and the vinyl dragging on the stylus mostly cancel each other out. Its direction is more or less towards the centre of the record. I’ve labelled this ‘D’ on the graphic.

That force would swing the stylus in towards to the centre of the record were there not, as continually demanded by Mr Newton, a force pushing it back out again. And what do you think is applying that force? Yes, full marks: the inner wall of the record groove. It applies a force equal to ‘D’ but in the opposite direction to ‘D’.

An anti-skating mechanism on a tone arm counteracts that force.

The way it’s usually done is by applying a counterclockwise ‘torque’ to the tone arm. Torque is the rotational equivalent of force. That’s often applied by using a small suspended weight on a thread to add a force in the same direction as ‘D’, but to the other side of the tone arm pivot. Sometimes various spring mechanisms are used. Infrequently, magnetic systems are employed.

No Anti-Skating Mechanism Is Exact
Manufacturers can calculate with great precision the amount of torque their anti-skating mechanisms will apply to the tone arm, and they can mark calibrations for various tracking weights. But all their efforts will nonetheless be at best approximate in cancelling out the force of the arm towards the centre of the record.

As I said earlier, the amount of friction, and therefore force on the stylus, depends not just on tracking weight but also on things such as groove modulation. But it also depends on angles, and those angles vary as the arm plays different sections of the record. A well-aligned cartridge will be at a precise tangent to the groove at two points throughout that travel. The rest of the time it will be off by a very slight angle. That changes the direction of the force on the stylus, so that in turn changes force ‘D’.

When I am setting up a turntable, I generally use the ‘groove-less’ track on my 1970s test LP, ‘An Audio Obstacle Course’, from Shure.

But even this applies in only one section of the record, and probably provides lower drag than a real track.

Anti-Skating not Required
As I suggested, there are turntable/tone arm designs where anti-skating is not required. One solution which was popular for a decade or more in the 1970s and 1980s was the linear tracking arm. Instead of the arm having a pivot, it was on some kind of rail so that it could slide smoothly at the back. As the record played, the whole arm would move slowly, remaining parallel to the current groove. Since the arm was always on a tangent to the groove, there were no misaligned angles. No misaligned angles means no misaligned forces. So no anti-skating was needed. Unfortunately, effective linear-tracking tone arms are difficult and expensive to build and can be temperamental so, in the end, radial, pivoting tone arms remained dominant, though some excellent linear tracking arms are available today.

The other possibility would be a tone arm with its pivot further out to the right, so that when it was playing the tracks half-way through an LP, it would align exactly with the track tangent. The main problem there is that for most feasible lengths of tone arm, there would still be misaligned forces, but in some positions the residual would be pushing inwards, and in others it would be pushing outwards. You’d still ideally have anti-skating, but it would somehow have to work both ways.

Unless that tone arm was really long. That would minimise that issue… but would introduce a stack more, including wear on the stylus and cantilever from having to push a higher-mass arm around, and likely low frequency resonances that could cause tracking problems. Plus, of course, the problem of the turntable chassis having to be large enough to accommodate such an arm.

Conclusion
I would recommend going with what the best turntable makers did at the peak of turntable development (I’d venture) in the 1980s: reduce the problem of misaligned forces with some kind of anti-skating mechanism. # Stephen Dawson

Don’t miss this special issue celebrating Australian Hi-Fi’s 50th year of publication, with a feature written by the magazine’s founder. The very first issue is now a collector’s item, and we think this 50th Anniversary issue will be too! Also full reviews of JBL’s L100 Classic speakers, B&W’s top-level DB1D subwoofer, Cambnridge Audio’s Azure 851N network player, and Gold Note’s IS-1000 amplifier, and all our regulars!

D’Agostino has released its Momentum HD Preamplifier. ‘This new Momentum HD is a major upgrade to the Momentum preamplifier,’ said designer Dan D’Agostino. ‘It adds HD status for the High Dynamic and High Definition improvements and incorporates substantial advancements in the power supply, audio circuitry, and control sections.’